The long-range goal of this research is to determine the effects of human aging on the regulation of blood flow to exercising skeletal muscles and its consequent impact on functional capacity. Our previous studies yielded novel results suggesting that the regulation of active muscle blood flow during large muscle exercise (leg cycling) is altered with age in healthy older women and men. Specifically, older women exhibit evidence of a significant limitation to vasodilation in the legs during submaximal exercise; whereas in older men there is evidence for augmented leg vasoconstrictor responsiveness. These sex-specific alterations could impair the ability of older adults to meet daily functional and metabolic demands. The goals of this revised competitive renewal are to investigate the central and local mechanisms responsible for these age and sex-specific alterations. Specific Aim 1 will determine the extent to which central mechanisms (cardiac output) limit leg perfusion during submaximal exercise in non-endurance trained older women and men. During conditions in which blood supply to the leg is limited centrally (leg cycling) or peripherally (single knee extensor exercise or leg cycling+augmented cardiac preload via saline infusion), we hypothesize that older women will exhibit a limit to leg vasodilation, whereas older men will demonstrate an altered balance between cardiac output and leg vasodilation, relative to their younger counterparts, respectively. Specific Aim 2 will define the local mechanisms responsible for sex differences in exercising leg hemodynamics with aging. This series of studies will examine structural and mechanical properties of the leg arterial vasculature (hypothesis 2), the effects of diminished (hypothesis 3) and enhanced (hypothesis 4A: ischemic handgrip, IHG) sympathetic modulation of leg hyperemia, endothelium-mediated vasodilation during exercise (hypothesis 4B; combined NO/PG blockade), and the interaction of these mechanisms in regulating leg exercise hyperemia (hypothesis 4C; NO/PG blockade + IHG). Based on preliminary results, we predict that women will have greater age-associated reductions in endothelium-mediated vasodilation and vascular compliance in the legs compared to men. We will investigate these aims by measuring systemic and leg blood flow (Doppler ultrasound), arterial pressure (femoral catheter or brachial cuff) and local oxygenation (NIRS) in younger (20-30 yr) and older (60-75 yr) men and women during leg exercise, with appropriate consideration for age and sex differences in leg muscle mass (DXA), quadriceps strength, and leg arterial structure (peak calf conductance, dynamic arterial compliance). The results of these studies will extend the findings from the previous award by defining the central and local mechanisms underlying age- and sex-differences in leg blood flow regulation during exercise, thereby setting the stage for future interventional studies aimed at optimizing muscle blood supply and functional capacity in older women and men.